1. Field of the Invention
This invention relates to a packaging material suitable for photographic photosensitive materials.
2. Description of Prior Art
Various types of packaging materials for photographic photosensitive materials have been put to practical use, and various properties are required according to their uses.
As a packaging material for photographic photosensitive materials, it is necessary to have various properties such as packaging material slitability, light-shielding, gas barrier, moistureproofness, antistatic property, rigidity, physical strength such as breaking strength, tear strength, impact puncture strength, Gelbo test strength and wear resistance, heat sealing properties such as heat seal strength, cut-seal ability, hot tack properties (hot-seal ability) and seal ability with other materials, flatness, slipping character and the like. The cut-seal ability is the slitability for fusing seal. In this sealing method, the hot plate for press is changed to a knife-shaped, and melting adhesion and cutting simultaneously occur.
Generally, it is difficult to satisfy these properties by a single material. Therefore, a single layer film of a high-pressure low-density polyethylene (LDPE) kneaded with carbon black or a pigment, and a composite laminated film composed of a LDPE film and a flexible sheet such as paper, aluminum foil or cellophane, etc. have been employed. An example of the conventional packaging film is shown in FIG. 10. This film is the most fundamental single-layer packaging material consisting of a light-shielding LDPE film layer 5a which is very thick. Another example of the conventional packaging film is shown in FIG. 11. This film was used for packaging a product which particularly requires moistureproofness, and it is composed of a light-shielding LDPE film layer 5a, a light-shielding metal foil layer 6 laminated on it through an adhesive layer 3, and a flexible sheet layer 4 laminated thereon through an adhesive layer 3. Still another example of conventional packaging film is shown in FIG. 12. This film was used as inner sheet of double-sheet gusset bag for packaging color photographic printing paper, and it is composed of a light-shielding LDPE film layer 5a, a light-shielding metal foil layer 6, a flexible sheet layer 4, and a light-shielding LDPE film layer 5a. They are laminated in that order, and an adhesive layer 3 is provided between each layer.
On the other hand, the present inventor has continued to investigate various packaging materials mainly for photographic photosensitive materials, and he has already disclosed the packaging materials of which physical strength is raised by combining two uniaxially stretched film layers (U.S. Pat. No. 4,331,725). With respect to the packaging material containing a low-pressure linear low-density polyethylene (L-LDPE), a packaging material comprising at least one light-shielding film layer of polyethylene polymer of which more than 50 wt. % is L-LDPE and containing more than 1 wt. % of a light-shielding material has been disclosed (U.S. Pat. No. 4,452,846). The packaging material of FIG. 13 is a fundamental packaging material composed of a light-shielding L-LDPE resin film layer 1a alone. The L-LDPE resin produced by copolymerization of ethylene with heptene-1 or octene-1 in liquid process having a density of 0.870 to 0.925 g/cm.sup.3 ASTM D-1505 and a melt index of 1.2 to 10 g/10 minutes at 190.degree. C. ASTM D-1238 was found to be excellent in physical strength, heat sealing properties, etc. It also has a small difference in tear strength between the longitudinal direction and the lateral direction. Based upon these findings, a light-shielding packaging material for light-sensitive materials has been developed comprising a layer of the polyethylene polymer film molded by the inflation process containing 40 to 99 wt. % of this resin having a thickness of more than 20 .mu.m and a ratio of tear strength in the longitudinal direction/tear strength in the lateral direction being more than 0.6 (U.S. Ser. No. 821,414). A L-LDPE resin film at least containing more than 40 wt. % of L-LDPE resin, 0.1 to 15 wt. % of carbon black and 0.03 to 1 wt. % of fatty acid amide lubricant was found to be excellent in slipping character without the occurrence of blocking. A packaging material for photographic photosensitive material containing this film as the inner surface layer has also been developed (U.S. Ser. No. 821,414). A packaging material has also been developed composed of a foamed sheet layer 4 and two light-shielding L-LDPE resin film layers 1a, 1a disposed on both sides thereof each through an adhesive layer 3 as shown in FIG. 14.
Physical properties of the conventional laminated films were not satisfactory except the films using L-LDPE resin, and during packaging, the films were sometimes torn or punctured or the heat seal of the films sometimes separated. In addition, when a large amount of a light-shielding material, such as carbon black was added, the physical strength of the film was lowered. Then, the amount was set at about 3 wt. %, and thickness of the film was more than 70 .mu.m. As a result, the film was stiff, and working efficiency of the packaging process was badly affected. The cost of the packaging was also expensive. For example, in the laminated film of FIG. 11, the metal foil layer 6 was incorporated in order to raise gas barrier, moistureproofness, antistatic properties and the like. As a result, tear strength, impact puncture strength and Gelbo test strength were lowered to the contrary, and in the case of packaging a heavy material, rupture became a problem. In the case of the packaging material of FIG. 12, though its physical strength was raised by laminating another light-shielding LDPE resin film layer 5a, the physical strength was still unsatisfactory. The laminated film disclosed in the specification of U.S. Pat. No. 4,331,725 was inferior in heat sealing properties, impact puncture strength and Gelbo test strength. Unstretched high-density polyethylene (HDPE) inflation film also had similar problems. In the case of the HDPE film containing more than 1 wt. % of carbon black, physical strength particularly tear strength in the longitudinal direction was low because of molecular orientation. Inflation moldability of this resin was not good. When the thickness of the film was thinner than 40 .mu.m, it was difficult to mold the inflation film having a lay-flat width of more than 50 cm. By employing L-LDPE resin for the packaging material, physical strength such as tear strength and Gelbo test, strength was raised, and the blending amount of a light-shielding material could be increased. However, for example, the light-shielding L-LDPE resin film disclosed in U.S. Pat. No. 4,452,846 was inferior in moistureproofness and gas barrier because of using low-density polyethylene resin. Moreover, when this film was employed for packaging a heavy photographic roll film having a sharp edge, black powder was occasionally generated by abrading the surface of the L-LDPE resin film with the edge of the roll film. Pinholes or breakage also were caused by the sharp edge. Besides, since tensile strength and Young's modulus were low, elongation or curling occurred when tension was added to the film during its molding, bag-making or packaging a product. The elongation also caused defects in light-shielding. Blocking and wrinkling of films were also liable to occur, and melt viscosity was high. Particularly, since inflation film moldability was inferior, various modifications of a molding machine were necessary.